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Atsuki TABO, Hisayoshi MATSUSHIMA, Takahiro OHKUBO, [Kei NISHIKAWA](https://orcid.org/0000-0002-7718-7606), Mikito UEDA

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[Arrangement of Al Ions between Ionic Liquid and Graphite Electrode Interface by AFM Force Curve Measurement](https://mdr.nims.go.jp/datasets/5a7002fb-af53-443a-bdd9-1e39261c4da2)

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untitledArticle Electrochemistry, 92(4), 043011 (2024)The 69th special feature "Frontiers of Molten Salts and Ionic Liquids"Arrangement of Al Ions between Ionic Liquid and Graphite Electrode Interfaceby AFM Force Curve Measurement†Atsuki TABO,a Hisayoshi MATSUSHIMA,a,§ Takahiro OHKUBO,b,§ Kei NISHIKAWA,b,c,§ and Mikito UEDAa,*,§a Graduate School of Engineering, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo, Hokkaido 060-8628, Japanb Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japanc National Institute of Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan* Corresponding author: mikito@eng.hokudai.ac.jpABSTRACTThe arrangement of ions at the interface between highly orientedpyrolytic graphite (HOPG) and Aluminum chloride–1-ehtyl-3-methylimidazolium chloride (AlCl3–EmImCl) ionic liquid wasevaluated using Atomic Force Microscopy (AFM) force curvemeasurements. In the force curve measurements, three stepswere observed towards the HOPG electrode. The width of eachstep was 0.3, 0.4, and 0.5 nm in order from the final arrival point.Their step widths were regarded as the thicknesses of EmIm+,Al2Cl7−, or their mixed layers. The force curve measurements ateach potential demonstrated that the width of the first layer closeto the HOPG tended to decrease as the potential shifted towardsthe less noble side.© The Author(s) 2023. Published by ECSJ. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY,http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium provided the original work is properly cited. [DOI:10.5796/electrochemistry.23-69151].Keywords : Ionic Liquid, AFM Force Curve Measurement, Ion Arrangement1. IntroductionIonic liquids and molten salts are liquids composed only of ionsand have high thermal stability, wide electrical window, and highelectrical conductivity. Because of these characteristics, ionic liquidsare used in various electrochemistry fields, such as the electro-deposition of metals and electrolytes for batteries.1–5 To understandthe phenomena or electrode reaction in electrochemistry, it isimportant to understand the structure of the solid-liquid interface.The widely known model for the electrode-electrolyte interface inaqueous solution is the Gouy-Chapman-Stern (GCS) model of theelectric double layer.6 Unlike aqueous solutions, ionic liquids do notcontain neutral liquids like water, thus it is difficult to apply thismodel. Fedorov et al. reported a model of the interface7 for the biasof ions near the interface of ionic liquids, and Mezger et al.experimentally confirmed the formation of layers of fluorinated ionicliquids at the charged sapphire interface by high-energy X-rayreflectometry.8 A rough model of the ionic species structure at theelectrode interface has also established from the results of the samepaper and simulation studies by Martin et al.9 and Ivaništšev et al.10When the electrode is positively charged, the anions are arranged onthe surface, followed by cations in the next layer, and gradually amixed layer of anions and cations is formed, leading to the bulk; aschematic illustration of each layer is shown in Fig. 1. The currentstructure of the interface at the ionic liquid/electrode has also beenproposed from in-situ observations by spectroscopy11 and ScanningTunneling Microscope,12 and the presence of arranged layers of ionsdifferent from the bulk has been reported. A commonly usedtechnique in the study of the solid/liquid interface of ionic liquids isAFM, which is an experimental technique for imaging surfacestructures using a small probe cantilever. Fukui reported detailedimaging of ionic structures at electrode interfaces using FrequencyModulation-Atomic Force Microscopy (FM-AFM).13In studies using AFM measurements, it is possible to measure thethickness of the ionic layer and the penetration intensity through thatlayer from the force curve measurements as well as imaging. Inforce curve measurements, the cantilever is brought close to theelectrode surface, and when it reaches a certain distance, thecantilever is pulled back. It is then deflected by the attractive/Figure 1. Schematic diagram of the electrode interface and ionicarray in aqueous solution/ionic liquid.³The contents include the authors’ presentation #P45 at 2023 JointSymposium on Molten Salts (MS12).§ECSJ Active MemberM. Ueda orcid.org/0000-0003-2068-1715ElectrochemistryThe Electrochemical Society of Japan https://doi.org/10.5796/electrochemistry.23-69151Received: December 21, 2023Accepted: January 14, 2024Published online: January 17, 2024Issued: April 1, 20241https://orcid.org/0000-0003-2068-1715http://creativecommons.org/licenses/by/4.0/https://doi.org/10.5796/electrochemistry.23-69151http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/https://orcid.org/0000-0003-2068-1715https://orcid.org/0000-0003-2068-1715https://orcid.org/0000-0003-2068-1715https://doi.org/10.5796/electrochemistry.23-69151https://doi.org/10.5796/electrochemistry.23-69151repulsive forces generated by the sample surface. The adsorptionforce and elasticity of the surface can be measured based on thisAFM deflection.Rob Atkin et al. evaluated the ion layer thickness, layer strength,and number of arranged ion layers of 1-butyl-1-methylpyrrolidi-nium tris(pentafluoroethyl) trifluorophosphate ([Py1,4][FAP]),1-ethyl-3-methylimidazolium tris(perfluoroethyl)trifluorophosphate([EmIm][FAP]), 1-Ethyl-3-methylimidazolium Bis(trifluorometh-anesulfonyl)imide ([EmIm][TFSA]), and 1-butyl-1-methylpyrrolidi-nium bis[(trifluoromethyl)sulfonyl]amide ([BMP][TFSA]) on thegold substrate surface.14,15 Fukui et al. reported on the magnitude ofthe force required to penetrate the solvation layer at the ionic liquid/rubrene single crystal interface by force curve measurements.16 Inthis technique, the cantilever penetrates the ionic layer formed at theinterface, resulting in a change in the graph as it approaches theinterface. In addition, a force change occurs when the cantileverpenetrates the ionic layer formed at the interface and reaches the nextionic layer. This change is observed as a step, rather than the gentlecurve it was before. Subsequently, the width of the ion layer and theforce required to penetrate it are measured. Cantilevers are change-sensitive probes and are therefore suitable for capturing changes inobjects such as ionic layers, which can be altered by minute forces.Changes in the ionic layer at the interface of ionic liquids depend onthe constituent ionic species, but also occur when an electric field isapplied. The application of electric fields to ionic liquids has beenconducted in the studies of solvents17 and lubricants18 in the field ofenergy storage, such as batteries and capacitors.19–21 The AFM forcecurve measurement used in this experiment allows in-situ observa-tion while applying an electric potential.Ionic liquids are also expected to be applied as electrolytes inelectroplating.22,23 A typical example is aluminum electroplating, inwhich many reports are of ionic liquids made by mixing 1-ehtyl-3-methylimidazolium chloride (EmImCl) and Aluminium chloride(AlCl3). The ionic liquid is in a liquid state near 25 °C, whichenables aluminum plating on low-melting-point substrate materials.In the AlCl3–EmImCl ionic liquid, the reaction of Eq. 1 proceeds.24AlCl3 þ EmImCl ! AlCl4� þ EmImþ ð1ÞFurthermore, the addition of excess AlCl3 generates the dimericcomplex ion of aluminum shown in Eq. 2.AlCl4� þ AlCl3 ! Al2Cl7� ð2ÞAl2Cl7¹ in this ionic liquid are electrodeposited as metallicaluminum at the cathode in the reaction of Eq. 3.244Al2Cl7� þ 3e� ! Alþ 7AlCl4� ð3ÞThe electrical conductivity and activity of ionic species in theAlCl3–EmImCl ionic liquid depend on the concentration of AlCl3.In aluminum electrodeposition, the ion required for Al electro-deposition is Al2Cl7¹ and this ionic species is formed at AlCl3concentrations above 50mol%. As reported by R. J. Gale et al.AlCl4¹ decreases and Al2Cl7¹ concentration increases as theconcentration of AlCl3 in the ionic liquid becomes above50mol%, and at 67mol% the Al ionic species alone can be treatedas Al2Cl7¹.25 Thus, the activity of Al2Cl7¹ is about 1 at an AlCl3concentration of 67mol%, and this composition is commonly usedin Al electrodeposition. In the 2 : 1 molar ratio AlCl3–EmImCl ionicliquid used in this experiment, only EmIm+ and Al2Cl7¹ ionsare present, so the AlCl3–EmImCl ionic liquid is denoted[EmIm][Al2Cl7] in the following.The reaction Eq. 3 shows that in the electrodeposition ofaluminum, Al2Cl7¹ receives an electron at the cathode; it remainsunclear how Al2Cl7¹ is reduced by receiving an electron whileEmIm+ is present close to the cathode. The authors hope to obtainthe AFM force curves with and without an applied electric field tounderstand the behavior of Al2Cl7¹ during electrodeposition.Based on previously reported AFM force curve measure-ments,14,15 this study uses force curve measurements at the interfacebetween highly oriented pyrolytic graphite (HOPG) and[EmIm][Al2Cl7] ionic liquid to evaluate the process of Al2Cl7¹reduction based on the arrangement of layers of ions at the interfaceand whether a potential is applied.2. ExperimentalPowdered anhydrous aluminum chloride AlCl3 (Kishida Chemi-cal Co., Ltd.) and 1-ethyl-3-methylimidazolium chloride (EmImCl;Tokyo Kasei Kogyo Co., Ltd.) were mixed at a molar ratio of 2 : 1with cooling. The electrolyte preparation was performed in a glovebox with an Ar atmosphere.After preparation of the ionic liquid, aluminum wires (99.99%,T 1.5mm, High Purity Science Laboratory) were immersed in theionic liquid to remove water and impurities from the ionic liquid andheated to 60 °C for at least seven days.Furthermore, to remove water from the ionic liquid, pre-electrolysis was carried out at 10mA for more than 3 h using anAl plate (99.99%, High Purity Science Laboratory) as the anodeand a glassy carbon plate (GC-20, Tokai carbon Co., Ltd.) as thecathode.Open circuit potential (OCP) and voltammogram measurementswere performed in a drying room with a room temperature of 22 °Cand a dew point of ¹60 °C.The experimental cell shown in Fig. 2 was used for theelectrochemical measurements and AFM observations. A HOPGplate (7 © 7mm) (ALLIANCE Biosystems) was used as theworking electrode, a Pt plate as the counter electrode, and an Alwire as the reference electrode. The HOPG plate was bonded to thecopper wire by silver paste. The surfaces of the silver paste andcopper wire were coated with epoxy adhesive to avoid contact withthe ionic liquid electrolyte. A potentiostat (HZ-7000, MeidenHokuto Corporation) was connected to the cell for the electrochem-ical measurements. Voltammogram measurements were performedat 22 °C, in the potential range from ¹0.5V to 0.5V (vs. Al/Al(III)),and at a sweep rate of 20mV s¹1.An AFM (SPM-9700HT, Shimadzu Corporation) was used forthe AFM measurements. The cantilever was a NANOSENSORSproduct with a spring constant of 0.2Nm¹1. The force curvemeasurements were performed at a pushing speed of 20 nm s¹1. Themeasurements were obtained at room temperature. The force curvemeasurements were performed at open circuit potential. Aftermeasurement at open circuit potential, measurements wereperformed in the chronoamperometry (CA) mode while applyingpotentials in the range from 0 to 710mV.The ion sizes of EmIm+ and Al2Cl7¹, the target of thisexperiment, were considered by computer simulation. The opti-mized geometry of EmIm+ and Al2Cl7¹ was determined using theDFT/B3LYP/6-311++G(2d,p) level of theory with the Gaussian16Figure 2. Cross-sectional schematic view of electrochemical cell;(a) HOPG working electrode, (b) Al counter electrode, (c) Alreference electrode, (d) Cantilever.Electrochemistry, 92(4), 043011 (2024)2package.26 The centroid for each ion was calculated based on atomicmass and coordinates, utilizing the standard orientation obtainedfrom Gaussian16 output, as depicted in Fig. 3. To gauge the size ofthe ion, the distance to the farthest atom from the centroid wascomputed. The lengths of the edges in each axis direction forthe standard coordinate system were also calculated to provideinformation about the degree of anisotropic form.The dimensions of the cuboid box were derived by consideringcovalent bond radii (H: 0.32 and Cl: 0.99 in ¡). Consequently, thedistance from the position of the end atom to the face of the cuboidbox was established to be equivalent to the covalent bond radius.The distances to the farthest atom from the centroid weredetermined to be 3.963¡ for EmIm+ and 3.906¡ for Al2Cl7¹. Interms of the cuboid box dimensions, the side lengths for EmIm+ andAl2Cl7¹ were (7.71, 5.31, 2.42) and (8.87, 5.67, 4.99) in ¡ forEmIm+ and Al2Cl7¹ in descending order. The computer-simulatedsizes of EmIm+ and Al2Cl7¹ ions are also shown in Fig. 3 assupporting data for discussion of the experimental results.3. Results and Discussion3.1 Electrochemical measurementThe results of the OCP measurements made in the cell in Fig. 2are shown in Fig. 4. The results show that the OCP was 620mV vs.Al/Al(III). This potential is considered to be due to the adsorption ofa small amount of water in the ionic liquid on the HOPG electrode.The results of the voltammogram measurements are shown inFig. 4b. The cathode current increased linearly from ¹100mV anddecreased linearly after reversing the potential, followed by theanode current. These currents correspond to the aluminum electro-deposition reaction and the dissolution reaction of the electro-deposited aluminum, respectively. Because the force curve measure-ments have to be obtained before the electrodeposition reactiontakes place, the potential on the low side was limited to 0V in thisexperiment.3.2 Force curve measurementsThe measured results of the force curve at OCP are shown inFig. 5a. Figure 5b is an enlarged view of Fig. 5a. The curve showsthe change in force when the cantilever is moved from the bulktowards the HOPG during push-in. The Y-axis indicates the forcedetected by the cantilever, and the X-axis indicates the position ofthe cantilever fixture, and not the position of the tip of the cantilever.Up to the 0 to ¹18 nm points on the horizontal axis, the position ofthe cantilever changes, but the value of the force does not change.This range is therefore considered to correspond to the bulk. When acantilever is pushed into the ionic layer in a force curve measure-ment, the position where the slope of the graph finally becomesFigure 3. The optimized ionic structure is depicted (left) EmIm+and (right) Al2Cl7¹. Black lines along with length in ¡ are thebox edges surround the ions in the standard coordinate systemconsidering the covalent bond radii of the atoms. The red dotted lineindicates the distance of the farthest atom from the centroid. Theions are represented by colored balls and sticks (Cl; light green, Al;silver, N; light purple, C; brown, H; white). The centroid position isindicated by a black ball.Figure 4. Electrochemical measurements in [EmIm][Al2Cl7] ionic liquid; (a) time variation of OCP of HOPG, (b) voltammogrammeasurement, (c) enlarged scale of the voltammogram.Electrochemistry, 92(4), 043011 (2024)3constant is defined as the final arrival point of the tip of thecantilever. In this figure, the slope of the graph is constant in therange of ¹30 nm to ¹50 nm, so this position is considered to be thefinal arrival point. It has been reported that the cantilever cannotpenetrate the strong ionic layer near the interface. Therefore, thefinal arrival point in this experiment is not on the HOPG plate, and isconsidered to be on the ionic layer that the cantilever could notpenetrate.14,27–29 Figure 6a shows the extrapolated line in the linearrange from ¹30 nm to ¹50 nm in Fig. 5a, extended in the y-axisdirection, and the difference between the extrapolated line (finalarrival point) and the measured force curve indicated as distance.The distance from the final arrival point is showed by the doublearrows in Fig. 5b. A magnified view of Fig. 6a is shown in Fig. 6b.In this figure, three obvious steps were observed before reaching thefinal arrival point. The width of each step was 0.3 nm, 0.4 nm, and0.5 nm from the final arrival point.If the needle tip of the cantilever cannot move deeper owing tothe resistance of the ionic layer, the position of the cantilever willnot change and the cantilever will be deflected. Therefore, in thenormal part of the graph, it extends diagonally up to the left. Whena certain amount of force is accumulated in the cantilever asdeflection, the probe penetrates the ionic layer. At this point, theforce applied to the cantilever does not change and it continues tomove until it penetrates the ionic layer.Unlike before that point on the graph, the x-axis value changesbut the y-axis value does not. Therefore, a confirmed step appearedon the graph. The width of the step is not different compared to theresults of the previous study, where the thickness of the EmIm+cation was 0.28 nm30 and the thickness of the Al2Cl7¹ anion was0.50 nm.31 The force curve measurements of the present experiment,as in the previous study of Atkin et al.14,15 allow us to measure theionic layer.In Fig. 6b, a slow change is observed between the ion layers, butthis change is not a gap between the ion layers. This change isthought to be due to the force accumulated in the cantilever asdeflection when the tip of the cantilever stops not being able topenetrate the ion layer. The force penetrating the ionic layer isconsidered to be stronger the closer it is to the electrode interface.Therefore, the deflection of the cantilever on the ionic layer near theelectrode interface is closer to the deflection of the cantilever at thefinal arrival point than the deflection of the cantilever against theionic layer near the bulk. This difference is the reason for thedifferent slopes of the rising graphs after penetrating the ionic layerin Fig. 6b.3.3 Force curve measurements under applying potentialNext, force curve measurements were obtained with potentialapplied to the electrochemical cell. The applied potential rangedfrom 0 to 710mV; as in the OCP measurement, a step was observednear the final arrival point even with the potential applied. Thethickness of the ionic layer and force required to penetrate the layerat each applied potential are shown in Fig. 7, with the layer closestto the final arrival point as the first layer and the layer following it asthe second layer. The error ranges were calculated based on standarddeviations. Figures 7a and 7b show that there was a significantdifference in the force required to penetrate the ionic layer betweenthe first and second layers. However, there was no difference in theforce due to potential. The difference in the force required toFigure 5. (a) Force curve during indentation on a HOPG electrode measured at OCP in [EmIm][Al2Cl7] ionic liquid and (b) an enlargedview of a part of Fig. 5a. (The position of the cantilever at the start of the measurement is set to 0.)Figure 6. Transformed force curve of Fig. 5, (a) relation between normal force and apparent separation and (b) an enlarged view of a part ofFig. 6a.Electrochemistry, 92(4), 043011 (2024)4penetrate shows that the first layer forms a stronger layer than thesecond layer.A change in thickness of the first layer was observed with theapplied potential, but not in the second layer. The formation ofa strong layer is affected by the density and denseness of theionic species within the ionic layer. Barbosa et al. reported thechange in stiffness of the ionic layer with applied potentialfor 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide([EmIm][TFSI]) electric bilayer structures at a charged tungstenoxide interface.32 Barbosa et al. stated that the increase in stiffnesswith applied potential is due to a change in the ionic orientation nearthe interface. He also reported that the stiffness of the ion layer wasincreased by the alignment of the ionic orientation angles owing tothe applied potential. Moreover, it was considered that the stiffnessof the ionic layer was increased by the increase in density of theionic species due to the applied potential. The change in the forcerequired to penetrate the first and second layers in this experimentmay be due to the greater interaction between the ions in the ionlayer because of the orientation angle of the ions. Because the firstlayer is closer to the interface than the second layer, it is alsoexpected to affect the structure of the ionic layer closer to theinterface.Baldelli et al. reported that HOPG and EmIm+ cations attracteach other by O-O stacking.15 This could facilitate the formation ofregular structures of ionic liquids at the interface. The structure ofthe ionic layer that forms a regular structure at the interface affectsthe ionic layers that are formed next to it. The influence coulddecrease as one moves away from the interface, and it is deducedthat as the ionic layer approaches the bulk, the orientation angles ofthe ions become more random. Therefore, the first layer closer to theinterface has a more aligned ionic orientation angle than the secondlayer, making the first layer more difficult for the cantilever topenetrate than the second layer.Figures 7c and 7d show that the thickness of the first layerchanged with the potential, but not with the second layer. Thethickness of the first layer decreased as one moved from the OCP tothe lower potential side.The decrease in the width of the ionic layer may be due to theattraction of ions near the bulk to the interface by the application ofelectricity. Because the ions are attracted to the interface, the ioniclayer containing the ions is also attracted to the interface. As theionic layer on the bulk side is attracted, the ionic layer on theinterface side is subjected to compression from the ionic layer on thebulk side. The stronger compression reduces the degree of freedomof the ion array angle of the ion layer and reduces the randomness ofthe arrayed ions. Therefore, it is assumed that the applied potentialbiases the arrangement of the ionic species to a specific angle,resulting in the densification of the ionic layer and a decrease in thelayer width. However, as shown in Figs. 7a and 7b, there was nopredominant difference in the change in the force required topenetrate the ionic layer owing to the applied potential. It is deducedthat near the bulk, even the potential right before electrodepositionwould have changed only to the decrease in the layer width. Anotherpossibility is that the ionic layer, which was measurable, may haveformed so densely that it could not be penetrated by the cantileverowing to the applied potential.If the formation of the dense layer is assumed to occur near theinterface, it is likely that more ion attracting occurs near theinterface, resulting in a reduction of the ion layer width. Assumingthat this occurs while considering the structure of the ions at theinterface, the following two mechanisms for the reduction ofAl2Cl7¹ are expected. One is to assume a cation-dominated,negatively charged electrode interface as shown in Fig. 1. Here,the application of potential decreases the thickness of each ioniclayer, bringing Al2Cl7¹ closer to the electrode, and providingelectrons from the electrode to Al2Cl7¹ by the tunneling effect,Figure 7. Results of force curve analysis under the applied potential; (a) potential change in the force required to penetrate the first layer,(b) potential change in the force required to penetrate the second layer, (c) potential change in the thickness of the first layer, (d) potentialchange in the thickness of the second layer.Electrochemistry, 92(4), 043011 (2024)5although EmIm+ is present between them. The Al2Cl7¹ that receivesthe electrons is reduced to metal aluminum and is deposited on theelectrode surface.Second, the ionic layer near the interface is not completely acationic layer, but anions are also present. Negatively chargedelectrode surfaces have a high amount of cations, but the possibilitythat anions may also be present has been reported in simulationsof ionic liquids other than the present study. Shan Zhou et al.performed simulations in graphite and [EmIm][TFSI] ionic liquids.33In their simulations, they assumed that TFSI¹ anions are present inthe first layer and that they are perpendicular to the electrodebetween the cations. Iwahashi et al. schematically modeled theinterface between the Pt electrode and 1-ethyl-3-methylimidazoliumbis(fluorosulfonyl)amide ([C2mim][FSA]) from electrochemicalmeasurements; when ¹1.0V was applied to the Pt electrode, someFSA¹ anions were present at the interface.34 However, anions arelikely to be present as part of the structure. A possible mechanism iswhen the Al2Cl7¹ anion receives electrons and aluminum depositionoccurs.4. ConclusionsThe layers of ions present at the interface between HOPG and[EmIm][Al2Cl7] ionic liquid were evaluated using AFM force curvemeasurements with or without applied potential.1. From the results of the force curve measurements, differentionic layer steps were observed at the interface between the[EmIm][Al2Cl7] ionic liquid and HOPG.2. These steps were 0.3, 0.4, and 0.5 nm, and were considered tobe arranged ionic layers.3. In the [EmIm][Al2Cl7] ionic liquid, the thickness of the ioniclayer decreased at the lower side when the force curvemeasurement was performed with a potential lower than thatof the OCP. The thickness of the ionic layer decreased at thehigher side when the force curve measurement was performedwith a potential lower than that of the OCP.AcknowledgmentsThis work was supported by a JSPS KAKENHI Grant Number21K04734.CRediT Authorship Contribution StatementAtsuki Tabo: Formal analysis (Lead), Investigation (Lead), Writing – original draft(Equal)Hisayoshi Matsushima: Conceptualization (Equal), Formal analysis (Equal)Takahiro Ohkubo: Formal analysis (Equal), Software (Equal)Kei Nishikawa: Formal analysis (Equal), Investigation (Supporting)Mikito Ueda: Conceptualization (Lead), Investigation (Supporting), Projectadministration (Lead), Supervision (Lead), Writing – review & editing (Lead)Conflict of InterestThe authors declare no conflict of interest in the manuscript.FundingJapan Society for the Promotion of Science: 21K04734References1. H. Liu, Y. Liu, and J. Li, Phys. Chem. Chem. Phys., 12, 1685 (2010).2. A. P. Abbott and K. J. McKenzie, Phys. Chem. Chem. 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